1. Field of the Invention
This invention relates to a method of producing active rice husk ash exhibiting a high degree of amorphousness and suitable for use as a concrete aggregate in place of fly ash, silica fume and the like.
2. Description of the Prior Art
In technology relating to rice husk and rice husk ash, the fundamental thinking has conventionally been first to consider rice husk as a heat source and second to try to find practical uses for the resulting rice husk ash. When rice husk is used a heat source, it is necessary to blow air into the rice husk after it is lit so as to achieve the highest combustion temperature possible. However, almost all of the rice husk ash obtained by such high-temperature burning is crystalline, very low in chemical activity and black in color. It therefore has few practical uses.
When air is blown into to the rice husk for burning it at high temperature, the resulting rice husk ash progressively crystallizes, a fact that can be readily confirmed from the X-ray diffraction pattern of the rice husk ash. The electrical conductivity of a saturated calcium hydroxide solution of the rice husk ash is very low, generally measuring around 0.3 mS/cm. Such rice husk ash cannot be expected to produce a significant effect when used as, for example, a pozzolan for concrete.
To be of high practical utility, moreover, a rice husk ash should preferably exhibit high chemical activity. It should also not be black. Since no method is available for low-cost industrial production of rice husk ash exhibiting high activity, there is a need for developing one.
At the Fourth CANMET-ACI International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete held in Istanbul, Turkey, between May 3 and May 8, 1992, the inventor on May 8 announced a method of using an ordinary muffle furnace as an electric furnace, charging 800 g of rice husk directly into the furnace, raising the temperature of the furnace from normal room temperature to 280.degree. C. and maintaining it at this temperature for 1.5 hr, raising the temperature of the furnace to 350.degree. C. and maintaining it at this temperature for 1.5 hr, raising the temperature of the furnace to between 400.degree. C. and 800.degree. C. and maintaining it at this temperature for 2 hr to conduct incineration, and removing rice husk ash from the furnace 5 hr later. Differently from rice husk ash obtained by directly igniting rice husk and burning it in a short time, the rice husk ash obtained by this method exhibited excellent chemical activity and was white in color. Since the method comprises two steps, the first for carbonization and the second for incineration, it will be referred to as the two-stage incineration method.
The object of the present invention is to provide a method for efficient production of white rice husk ash exhibiting chemical activity which is based on this two-stage incineration method.